...How a system behaves when it doesn't have backflow prevention in place...
That's like asking how a vehical behaves when it doesn't have seatbelts. Everything is fine until you get into an accident.
The purpose of a backflow preventor is to insure that water, possibly contaminated with soil, dirt, fertilizer, and dog poop, does not find it's way back into the drinking water supply. WITH an backflow preventer, the system will loose a little bit of pressure (3psi-15psi depending upon the type of backflow required in your area). Now lots of people will think a backflow preventor isn't needed, after all the "contaminated water" is on the far side of an automatic sprinkler valve, with the water on the other side of the valve under pressure. There's no way for that water to flow back before the valves. But that just isn't the case. Things can happen that result in backwards pressure, sometimes just momentaraly, that can litterally suck water backwards pulling that contaminated water back before the valves. The purpose of a backflow preventer is to STOP that backflow of water under those conditions. Without a backflow preventor, your irrigation system will work fine. You might get sick drinking the water in your house... but the irrigation system will work fine.